This invention relates to an image deleting agent for offset printing plates which use silver images produced on the offset printing plates as ink receptive areas which deletes the ink receptivity of the silver images by rendering hydrophilic the undesired portion of the silver images. More particularly, it relates to removal of ink receptivity of undesired portion of silver images photographically or physically formed on offset printing plates by rendering completely hydrophilic that portion of the silver images without giving any adverse effect on printing characteristics and properties of substrate by treating the surface with a specific treating agents at an optional stage of printing procedure. Usually, such treatment is called "deletion" in the field of printing and so the inventors use this terminology in this specification. The treating fluid used for deletion is called "deletion fluid".
Generally, portions of printing plate made by photomechanical process which require deletion include those which are already present in originals to be copied, produced at the time of exposure or produced due to uneven developing treatment, etc.
That is, for example, there are cases where the originals contain undesired images or have dusts, dirty powders, finger prints and other stains and furthermore, when laminated originals are used, a shadow may be produced by the exposure. In order to obtain good printed matters, said portions must be deleted at the stages of plate making and printing. Generally, the deletion is an essential treatment and so it is preferred that necessary deletion can be carried out at an optional stages in plate making and printing procedures.
Many proposals have been made for deleting unnecessary images of offset printing plates which use silver images as ink receptive areas.
For example, the compounds having mercapto or thion group and a hydrophilic group which are disclosed in Japanese Patent Laid-Open Application (Kokai) Nos. 92101/73, 21901/76, 53002/79 and 9661/84 can make hydrophilic the silver images certainly and by almost instantaneous reaction. However, they have disadvantages such as bad odor, adverse effect on human bodies due to strong acidity and poor in storage stability. Thus, further improvement has been demanded.
The deletion fluid containing a ferricyanide or a metal salt of EDTA and a thiocyanate which is disclosed in Japanese Patent Laid-Open Application (Kokai) No. 41808/73 has the problems such as environmental pollution, gradual recovery of ink receptivity by printing after deletion which causes scumming and poor storage stability.